Methods of installing offshore platforms

ABSTRACT

A tension leg subsea platform (TLSP) ( 1 ) comprises a buoyant, submersible hull anchored to the seabed by tendons ( 3 ). A jack-up platform ( 6 ), either for drilling operations, production operations or any combination of these, is located such that its legs ( 8 ), when lowered, rest on the TLSP. In an alternative arrangement (FIG.  3 ), legs or columns are pre-installed on the TLSP, and the working platform lifted or floated onto them. In either case, the two components can be fabricated and travel separately to the field location. The effective depth of the TLSP may be 30 m to 100 m, while the sea itself is substantially deeper than 100 m. The working platform may be re-used at other locations. A drilling platform may be exchanged for a production platform while the TLSP remains moored on location, and the production platform may in turn be exchanged for a decommissioning platform.

[0001] The present invention relates to methods of installing anoffshore platform, such as may be used for drilling, production and/ormaintenance operations, and to methods of constructing, deploying andusing such platforms. The invention further relates to platformsinstalled by such a method.

[0002] Offshore drilling for oil or gas requires a platform to act as aworking stage for conducting offshore operations. There are severaltypes of platform used in such operations, choice of which depends on anumber of factors, such as the role of the platform, likely weatherconditions, permanence of the platform and the depth of the water.

[0003] Conventional offshore platforms are permanently installed on asteel structure referred to as a “jacket”, which is erected on theseabed, and has a deck and topsides attached for the necessary equipmentand accommodation. For other situations, especially in deep water, andfor temporary operations, floating or “semi-submersible” vessels anddrillships are also used.

[0004] A different type of platform is the jack-up platform which hasbeen used for offshore drilling for some time now. A jack-up platform isa mobile drilling platform wherein the platforms legs can be raisedwhile under tow, projecting only a few metres below the water surface,and therefore can be easily moved to a drilling/production location.This has the advantage that the deck and topsides can be assembled onland or at least in shallow waters, and then towed as a unit to theirdesired location. When at the desired location, the legs can be loweredto the seabed and therefore, in use, the platform has greater stabilitythan many other types of mobile platforms. Jack-up platforms, however,are only suitable for use in shallow water applications, generally lessthan 110 metres deep. One example is described in U.S. Pat. No.4,907,912, which also has a submersible production storage barge.

[0005] Another type of known platform is the tension leg platform orTLP. The TLP is a light, buoyant platform moored to the seabed by somemeans, such as wires, cables, pipes or chains, such that these mooringsare under tension due to the buoyancy of the platform. The advantages ofthis type of platform are that it can be used at virtually unlimiteddepths, is easily constructed, and is mobile and therefore can be towedto a drilling/production location.

[0006] When on-location, vertical wave motions should preferably bereduced as much as possible ensuring that the moorings are not underundue tension and therefore reducing the necessary strength and cost ofthe moorings. This is best achieved by having the buoyancy well belowthe surface of the water. However, this would result in a tall structureand therefore when being towed, such a tall structure would be veryunstable, although stability is key when floating to location.

[0007] Another type of platform which may address this drawback isproposed in U.S. Pat. No. 4,913,591 (Steele). Here, a submersible,buoyant pontoon is secured by tendons to the seabed in the manner of atension leg platform. The pontoon is provided with a column or columnsextending upwardly from it, and passing through the upper, operatingdeck. The upper deck can thus be raised and lowered above the surface ofthe sea in the manner of a jack-up rig. U.S. Pat. No. 4,604,001(Wetmore) describes a “jack-down” platform, in which four legs areanchored to the seabed via tendons. The legs pass through theextremities of a buoyant hull, and the hull is jacked down the legs, tosubmerge it and apply the tension which creates a tension legarrangement.

[0008] It is an object of the invention in a first aspect to allow useof existing, economical, field-proven technology for the top structureof drilling and/or production facilities, independently from waterdepth.

[0009] The inventors have realised that it is possible effectively toraise the deep water level to that of the shallow water level, by usingone form of working platform, such as a jack-up, on top of another type.In particular, the invention in a first aspect provides a method ofinstalling a working platform in a depth of water, the methodcomprising:

[0010] providing a buoyant, submersible hull for subsea installation,the submersible hull being adapted to provide a foundation for a workingplatform standing above the surface of the sea

[0011] anchoring the hull to the seabed at a desired depth and locationby means of tendons, thereby to form a tension leg subsea platform(TLSP), and

[0012] subsequently mounting a working platform on the TLSP.

[0013] The TLSP may be moored to the seabed by some means, such that theworking platform itself is operated at a water depth conventionallysuitable for a fixed platform (typically 30 m-100 m), while the buoyancyof the TLSP is well below the water surface. The effective depth of theTLSP may be 30 m to 100 m, while the sea itself is substantially deeperthan 100 m.

[0014] The working platform may in particular be a jack-up platform,although this is not essential. A jack-up platform, either for drillingoperations, production operations or any combination of these, islocated such that its legs, when lowered, rest on the TLSP. In analternative arrangement, legs or columns are pre-installed on the TLSP,and the working platform lifted or floated onto them. In either case,the two components can be fabricated and travel separately to the fieldlocation.

[0015] The invention in suitable embodiments enables fabrication of aplatform for deep water use in yards with relatively shallow depth ofwater to work in, such as 5-10 metres. In particular, the inventionprovides a method in which the TLSP and working platform are separatelyfabricated, the TLSP submerged and moored at a desired location, and theworking platform then mounted so as to be supported above the surface bythe TLSP.

[0016] Since the height of the individual components travelling to thelocation can be made much smaller than their depth when on location, theinvention in suitable embodiments permits an increase in the proportionof work that can be conducted in yards local to the offshore site.

[0017] The invention in a second aspect provides a method of installinga working platform in a depth of water, the method comprising:

[0018] providing a buoyant, submersible hull for subsea installation,the submersible hull being adapted to provide a foundation for a workingplatform standing above the surface of the sea

[0019] anchoring the hull to the seabed at a desired depth and locationby means of tendons, thereby to form a tension leg subsea platform(TLSP), and

[0020] mounting a first working platform on the TLSP

[0021] and wherein said working platform is exchanged for at least oneother working platform while the TLSP remains moored.

[0022] This allows use of the same substructure (TLSP) for the totallifecycle of the project, and to change out as necessary, thefloating/jacked up/ minimal topside structures with necessary equipmentfor, for example, drilling purpose, work area purpose or productionpurpose.

[0023] The inventors have further realised it is possible to use thesame TLSP substructure for the total lifecycle of a project.

[0024] This method of performing offshore operations is achieved byexchanging, as necessary, the floating/jacked-up/minimal topsidestructures with the necessary equipment for the operation beingundertaken. The TLSP can remain in place throughout.

[0025] The working platform can be of the same traditional design as forshallower water operation, and may even be one previously used on theseabed, and/or on another TLSP.

[0026] A preferred embodiment of the invention has a well-head locatedat the TLSP level such that the TLSP effectively acts as a seabed forthe jack-up platform. However, other embodiments of the invention mayhave the well-head located either at the seabed or the jack-up platformlevel.

[0027] A preferred embodiment in both aspects of the invention has theworking platform coupled to the TLSP by means of at least oneretractable dog.

[0028] The first and second aspects of the invention may be performedindependently of one another, or may be combined in the same method.

[0029] The invention yet further provides platforms installed by themethods according to the invention, as defined in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Embodiments of the invention will now be described, by way ofexample only, by reference to the accompanying drawings, in which:

[0031]FIG. 1. shows a jack-up platform afloat on the water surface and aTLSP below the water surface, in the course of deploying a platform inone embodiment of the present invention.

[0032]FIG. 2. shows the same jack-up platform located on the TLSP afterdeployment.

[0033]FIG. 3. Shows a further embodiment of the invention wherein thelegs are pre-installed on the TLP.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0034]FIGS. 1 & 2 show one embodiment of the present invention. FIG. 1shows a tension leg platform TLSP 1 comprising a submersible, buoyanthull moored to a base 2 by tendons 3. The base 2, located in thevicinity of at the seabed (level (a)), may be positioned and maintainedin position by any suitable technology, such as weight or suctionanchors or other means, depending on the soil condition and the loadsinduced by the environment. A unitary base is not essential. Flexible orrigid conduits, pre-installed to the TLSP 4, transport oil, gas, anywell production or any other fluids required for the process in hand,from the seabed to a well-head 5 on the TLSP. In the vicinity of thewater surface (level c) there floats a jack-up platform 6 with raisedlegs 8. The jack-up can be of known type, for example having production,drilling and accommodation modules 9, the nature and design of whichdepends on what application the platform 6 is to be used for.

[0035] The TLSP 1 is either manufactured from a buoyant material or hasfloats attached to it, which ensures that the tendons 3 are kept undertension. The tendons 3 are of such a length that the TLSP 1 is held at awater depth of 30 m-100 m (level b)), the buoyancy of the TLSP istherefore significantly below the water surface c). It should be notedthat the strain is being taken entirely by the tendons 3, the conduits 4are not themselves under any tension, other than their own weight andany transmitted weight by friction.

[0036] The self-elevated jack-up platform 6 floats upon the watersurface c) while being towed to the drilling site. The jack-up platformhas tubular or lattice legs 8 which can be raised or lowered by electricor hydraulic jacks, winches or other means (not shown). When in transitthe legs 8 are raised so as not to drag in the water. By this means, thejack-up platform can be fabricated inshore, at any depth, with completedeck and topsides (modules 9).

[0037]FIG. 2 shows the invention when the jack-up platform 6 is in placewith transfer lines 10 and control and command function lines 11attached. Once the platform 6 is floating directly above the TLSP 1 thelegs 8 of the jack-up platform are lowered to rest on the TLSP 1 andthen further jacked such that the platform 6 is raised clear of thewater surface 7. Transfer lines 10 (which may also be rigid or flexible)are then lowered from the platform 6 or installed by external means andequipment and are attached to the well-head 5, so as to act as conduitsfor the oil, gas or any other fluid either being produced or requiredfor the process from well-head 5 to surface platform 6. The well-head isnow at level b), a shallow water level, and therefore the coupling ofthe transfer lines 10 to the platform 6 is a simpler operation. Thecontrol and command function lines 11 are for the control of anyequipment located subsea, at either level b) or a), such as thewell-head 5, manifold or ballasting of the TLSP 2.

[0038] Depending on the actual conditions, other embodiments may havecables, pipes, chains, tethers as the tension members of the TLSP 1instead of the tendons 3. Another embodiment may have conduits 4 thatare installed by jack-up as opposed to being pre-installed, and otherembodiments of the invention may have the well-head located at levels a)or c).

[0039]FIG. 3. shows an embodiment of the invention wherein the legs 12of the top platform 13 are now pre-installed on the TLSP 1 such that thetops of the legs 12 are at a level usually just above the surface of thewater. (The ends of said legs can alternatively be located below thewater level.) The transfer lines 7 are pre-installed on guides 14 forsimpler coupling to the top platform 13. The TLSP is unchanged belowlevel b) but, in this example, the top platform is lifted by some meansonto the pre-installed legs 12. Instead of the guides 14 the transferlines 7 may be linked to the pre-installed legs 12.

[0040] In these two embodiments, it is seen how a conventional jacket orjack-up platform design can be used in a variety of applications, thedepth of water below the design depth of the legs being spanned by theTLSP.

[0041] The skilled reader will appreciate that many variations arepossible within the spirit and scope of the invention defined in theappended claims, and the embodiments disclosed herein should be regardedas examples only.

[0042] The TLSP may be formed by anchoring the submersible hull

[0043] It will be understood that terms such as “marine” and “seabed” asused in the description and claims are not intended to excludeapplication in bodies of water other than open seas. “Legs” is usedbroadly to refer to any form of support column, which may be a steellattice, concrete or other suitable construction. Depending on what loadis to be supported, a single column may in principle be sufficient, asshown in the first embodiment of U.S. Pat. No. 4,913,591, mentionedabove.

1. A method of installing a working platform in a depth of water, themethod comprising: providing a buoyant, submersible hull for subseainstallation, the submersible hull being adapted to provide a foundationfor a working platform standing above the surface of the sea anchoringthe hull to the seabed at a desired depth and location by means oftendons, thereby to form a tension leg subsea platform (TLSP), andsubsequently mounting a working platform on the TLSP.
 2. A method asclaimed in claim 1, wherein said working platform is a jack-up platformcomprising one or more supporting columns and a working deck mounted onsaid column(s) for vertical travel thereon, said mounting beingperformed subsequently to anchoring of the TLSP by lowering the columnsfrom the working platform until they are planted on the TLSP.
 3. Amethod as claimed in claim 1 or 2, wherein said working platform isoperated as a drilling platform.
 4. A method as claimed in claim 1 or 2,wherein said working platform is operated a production platform.
 5. Amethod as claimed in claim 1 or 2, wherein said working platform isoperated as a decommissioning platform.
 6. A method as claimed in anypreceding claim, wherein said working platform is exchanged for anotherworking platform while the TLSP remains moored.
 7. A method as claimedin any preceding claim, wherein said working platform is one which hasoperated at another location prior to installation on said TLSP at itspresent location.
 8. A method as claimed in any preceding claim, whereinthe effective depth of the TLSP is in the range 30 m to 100 m, while thesea at said location is substantially deeper than 100 m.
 9. A method asclaimed in any preceding claim, wherein the TLSP carries wellheads forconnecting the working platform to seabed wells.
 10. A method as claimedin any preceding claim, wherein the working platform is coupled with theTLSP by means of members movable to engage and disengage formations onthe TLSP.
 11. A method of installing a working platform in a depth ofwater, the method comprising: providing a buoyant, submersible hull forsubsea installation, the submersible hull being adapted to provide afoundation for a working platform standing above the surface of the seaanchoring the hull to the seabed at a desired depth and location bymeans of tendons, thereby to form a tension leg subsea platform (TLSP),and mounting a first working platform on the TLSP and wherein saidworking platform is exchanged for at least one other working platformwhile the TLSP remains moored.
 12. A method as claimed in claim 11,wherein said first working platform is mounted subsequently to anchoringand submerging the hull to form said TLSP.
 13. A method as claimed inclaim 11 or 12, wherein each of said working platforms is of ajack-upplatform type comprising one or more supporting columns and a workingdeck mounted on said column(s) for vertical travel thereon, saidmounting being performed subsequently to anchoring of the TLSP bylowering the columns from the working platform until they are planted onthe TLSP.
 14. A method as claimed in claim 11, 12 or 13, wherein one ofsaid working platforms is operated as a drilling platform.
 15. A methodas claimed in any of claims 11 to 14, wherein one of said workingplatforms is operated a production platform.
 16. A method as claimed inany of claims 11 to 15, wherein one of said working platform is operatedas a decommissioning platform.
 17. A method as claimed in any of claims11 to 16, wherein said working platform is one which has operated atanother location prior to installation on said TLSP at its presentlocation.
 18. A method as claimed in any of claims 11 to 17, wherein theeffective depth of the TLSP is in the range 30 m to 100 m, while the seaat said location is substantially deeper than 100 m.
 19. A method asclaimed in any one of claims 11 to 18, wherein the TLSP carrieswellheads for connecting the working platform to seabed wells.
 20. Amethod as claimed in any one of claims 11 to 19, wherein each workingplatform couples with the TLSP by means of at least one retractable dog.21. An offshore platform installed by a method as claimed in anypreceding claim.
 22. An offshore platform comprising: a tension legsubsea platform (TLSP) comprising a buoyant, submersible hull for subseainstallation and tendons anchoring the hull to the seabed at a desireddepth and location, and a working platform standing above the surface ofthe sea, the submersible hull providing a foundation for the workingplatform, said working platform being one which has operated at anotherlocation prior to installation on said TLSP at its present location. 23.An offshore platform comprising: a tension leg subsea platform (TLSP)comprising a buoyant, submersible hull for subsea installation andtendons anchoring the hull to the seabed at a desired depth andlocation, and a working platform standing above the surface of the sea,the submersible hull providing a foundation for the working platform,said working platform being one which has replaced another workingplatform installed previously on said TLSP at its present location.